This invention relates to casting apparatus and to a method of casting for use in casting metals, and in particular but not exclusively for use in casting low melting point alloys into a die. The casting of such alloys is used for example to produce cores which may be subsequently used in moulding plastic material in which the alloy is subsequently removed from the moulded plastic component by melting the core.
It is proposed in GB-2063127 to provide casting apparatus in which a die is supplied with molten alloy under pressure from a cylinder and piston arrangement via a three-way valve which, on the return stroke of the piston, is actuated to connect the cylinder with a supply of molten metal for recharging the cylinder. A disadvantage of such a valve is that its operation relies on a valve member movable within a valve chamber and guided by sliding contact between the member and the chamber walls, leading to the problems of wear, misalignment and possible jamming of the valve.
According to the present invention there is disclosed casting apparatus comprising a dispenser defining a dispensing chamber of variable volume receiving molten metal in use, a valve having a housing defining a valve chamber and a passageway communicating between the valve chamber and the dispensing chamber, the valve further defining an inlet port communicating with a supply of molten metal and further defining an outlet port communicating with a die, a valve member located in the valve chamber, actuating means selectively operable to move the valve member into a first position closing the inlet port and a second position closing the outlet port whereby in the first position the valve connects the dispensing chamber with the die for casting metal and in the second position the valve connects the dispensing chamber with the supply of molten metal for recharging the dispensing chamber, the valve including first and second annular seats surrounding the inlet and outlet ports respectively and engageable by respective first and second annular sealing surfaces of the valve member in the first and second positions respectively and wherein the actuating means comprises a stem fixed to the valve member, a linear actuator operable to provide reciprocating axial movement of the stem and rotating means operable during axial movement of the stem to rotate the stem so as to vary the relative contact position of the respective seat and sealing surfaces.
An advantage of such an arrangement is that the angular position of the valve member relative to the valve housing can be indexed through angular increments (other than multiples of 360.degree.) such that repeated seating contact between the same points is avoided. Both the sealing surfaces and the respective seats will then tend to wear evenly, such that leak-free sealing is maintained. The indexing also tends to prevent the build-up of solid material on the sealing faces and seats.
Conveniently, the rotating means comprises a collar rotatably mounted on the stem, means rotating the collar in a first direction during axial movement of the stem in a first direction and in a reverse direction of rotation during axial movement of the stem in a reverse direction and ratchet means operable between the collar and the stem to transmit rotational movement to the stem in one direction only.
Preferably each of the sealing surfaces is profiled so as to make substantially line contact with the respective seat.
An advantage of such line contact is that a substantially leak-free seal is provided and the build-up of deposits on the respective sealing surfaces is prevented.
Advantageously the profile of each of the first and second sealing surfaces is substantially that of a segment of a sphere.
An advantage of this profile is that it facilitates self-alignment of the valve member on making sealing contact and enables line contact to be made between the sealing surfaces and the respective seats.
Preferably the valve chamber is bounded by chamber walls which are spaced from the valve member by circumferentially spaced guide means projecting from the chamber walls into sliding contact with the valve member to maintain the valve member in axial alignment during travel of the valve member between the first and second positions.
Preferably the valve housing is oriented in use such that the inlet port is vertically above the outlet port and the apparatus includes means biassing the valve member downwardly into the second position such that the outlet port is automatically closed if the valve actuating means fails to operate.
An advantage of such an arrangement is that outflow of molten metal from the outlet port of the valve is prevented under conditions of failure of the valve actuating means. This is particularly important when it is necessary to disconnect the die from the apparatus in which case failure of the valve actuating means might otherwise result in spillage of molten metal.
Advantageously the valve includes a further port communicating with a pressure relief valve operable to limit pressure of metal in the valve to a predetermined limit.
This is particularly advantageous where the dispenser includes for example a hydraulic actuator capable of generating excessive pressures under certain conditions. The pressure relief valve enables the pressure of molten metal supplied to the die to be kept below a predetermined limit.
The casting apparatus may include a dispenser which comprises one or more cylinders having a cooperating piston and means actuating the piston to vary the volume of the dispensing chamber constituted by the cylinders.
According to a further aspect of the present invention there is disclosed a method of casting by means of apparatus comprising a dispenser defining a dispensing chamber of variable volume receiving molten metal in use, a valve having a housing defining a valve chamber and a passageway communicating between the valve chamber and the dispensing chamber, the valve further defining an inlet port communicating with a supply of molten metal and further defining an outlet port communicating with a die, a valve member located in the valve chamber, actuating means selectively operable to move the valve member into a first position closing the inlet port and a second position closing the outlet port whereby in the first position the valve connects the dispensing chamber with the die for casting metal and in the second position the valve connects the dispensing chamber with the supply of molten metal for recharging the dispensing chamber, the valve including first and second annular seats surrounding the inlet and outlet ports respectively and engageable by respective first and second annular sealing surfaces of the valve member in the first and second positions respectively and wherein the actuating means comprises a stem fixed to the valve member, a linear actuator operable to provide reciprocating axial movement of the stem and rotating means operable to rotate the stem, the method including the step of rotating the valve member relative to the valve housing during axial movement of the valve member in at least one direction of axial movement so as to vary the relative contact position of the respective seat and sealing surfaces.
Embodiments of the present invention will now be described by way of example only and with reference to the accompanying drawings.